<Abbreviations>
3GPP3rd Generation Partnership ProjectBSBase stationCATCategoryCDMAcode division multiple accessCONSDisadvantagesCRCCyclic Redundancy checkCWICCodeword level successive interference cancellationDCIDownlink control informationDCI-EEnhanced Downlink control information assisting MUSToperationDCI-MDown Link Control Information carrying assistantinformation for MUST data signal reception and decodingDLDownlinkeNBEnhance NodeBePDCCHEnhanced Physical Downlink Control ChannelFDMAfrequency division multiple accessIPsIntellectual propertiesL-CWICLinear codeword level successive interference cancellationLTELong Term EvolutionLTE-ALTE-AdvancedMLMaximum LikelihoodML-CWICMaximum likelihood codeword level successiveinterference cancellationMUSTMulti User Superposition TransmissionOFDMAOrthogonal Frequency Division Multiple AccessPDCCHPhysical Downlink Control channelPDSCHPhysical Downlink Shared channelPROSAdvantagesRANRadio Access NetworkRATRadio Access technologyREResource ElementRIRank IndicatorR-MLReduced complexity maximum likelihoodRNTIRadio Network Temporary IdentifierRRCRadio Resource controlSC-FDMASingle Carrier-FDMASICSuccessive Interfence CancellationSINRSignal to Interference plus Noise ratioSLICSymbol level interference cancellationSNRSignal to Noise ratioTDMATime Division Multiple AccessUEUser equipmentULUplink
It is anticipated that mobile traffic will increase drastically in the coming years, and some estimate that mobile traffic will increase more than 500 fold in the coming decade. In order to cater for this massive increase in mobile traffic, new solutions that increase the capacity of mobile networks are required.
An important aspect of improving system capacity in cellular communication has been the design of cost-effective radio access technologies (RATs). Typically, RATs are characterised by multiple access schemes, such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) and orthogonal-frequency division multiple access (OFDMA), each of which provides means for multiple users to access and share system resources simultaneously.
Current mobile communication systems such as Long-Term Evolution (LTE) and LTE-advanced employ OFDMA for downlink (DL) and single carrier (SC)-FDMA for uplink (UL). The use of OFDMA in LTE enables good system capacity, while retaining a simplified receiver design. Technically, a more advanced receiver design would enable a higher transmission rate, and thus an improved bit rate per channel (i.e. time-frequency unit), boosting spectrum efficiency or spectrum utilisation.
It has been established that superposition coding transmission, together with advanced interference cancellation, can be used to achieve capacity on a Gaussian broadcast channel. Superposition coding is a non-orthogonal scheme which allows multiple users with considerably different SINRs (Signal to Interference and Noise ratios) to share the same resources (i.e. time and frequency resources such as LTE REs) without the needs of spatial separation. Due to its capacity achieving nature, superposition coding mechanisms have been identified as a candidate RAT for new air interfaces in 3GPP 5G networks, and have been endorsed for feasibility studies in 3GPP RAN. In principal, superposition coding or multiuser superposition transmission (MUST) may optimally exploit the channel ordering or the path loss difference of paired users served by the same transmission point.
As illustrated in FIG. 1, a UE 1 who is geographically closer to the base station has a higher channel gain or higher SINR than a UE 2 who is geographically far away from the base station. As such, a downlink transmission that can be decoded at the far-UE (UE 2) can possibly be decoded at the near-UE (UE 1), but not vice versa. Conceptually, the DL transmission power to the far-UE is considerably higher than the DL transmission power to the near-UE, to account for higher path loss.
MUST takes advantage of this considerable transmission power difference by superimposing the downlink transmissions for the near-UE (with low transmit power) in to that for the far-UE (with high transmit power) and transmitting the superimposed or composited signal in the same set of channel resources achieving multiple access gain in the power domain.
Due to the transmission power difference, the signal of the near-UE (UE 1) hardly reaches the far-UE (UE-2) and desirably appears as the noise at the far-UE (UE-2). This allows the far-UE (UE 2) to decode its signal in the traditional way. Since the near-UE (UE-1) has a high channel gain, it can receive and decode far-UE's signal, and cancel or remove the far-UE's signal from the received signal to decode its own signal. This procedure at the near-UE is called successive decoding or successive interference cancellation (SIC).
In order to cancel or remove a signal from a far-UE, or jointly detect and decode a signal of a near-UE with the presence of a far-UE signal, a near UE needs to know some information about the signals of the far UE. According to 3GPP LTE, signal information of a far-UE may be transmitted from a base station as downlink control information (DCI).
Recently, 3GPP RAN-WG1 concluded a feasibility study on MUST with the recommendation of 3 potential MUST categories 1, 2 and 3, and recommendation of candidate receiver schemes for near-UEs including a maximum likelihood (ML) receiver, a reduced complexity maximum likelihood (R-ML) receiver, a symbol level interference cancellation (SLIC) receiver, a linear codeword level successive interference cancellation (L-CWIC) receiver, and a maximum likelihood codeword level successive interference cancellation (ML-CWIC) receiver.
Technically, each of above recommended MUST categories have their own pros and cons, and flexibility, which depends on a base station (eNB) implementation. Therefore, one or more than one or all above mentioned MUST Categories may be endorsed by 3GPP RAN for use as declared implementation options.
Furthermore, receiver schemes generally differ between UE manufacturers, and receiver schemes are generally not mandated by 3GPP. As such, the scheme of a receiver is not known by the servicing base station (eNB) for special services, for additional information, signalling assisting advanced signal reception and/or decoding at the UE receiver, which may prevent proper reception, detection and decoding of a MUST signal at a near-UE.
In order to allow for UE receiver evolution, it is desirable to allow multiple UE receiver schemes to coexist at the same servicing cell/BS. Additionally, it is desirable to supporting different MUST categories at a BS, independent of the near-UE receiver schemes.
Furthermore, due to different downlink data services required by far-UEs and paired near-UE, MUST data signals are generally not required to be transmitted on all scheduled downlink subframes.